1. Field of the Invention
The present invention relates to an optical information detection method, and relates to an improvement in the S/N ratio of a readout signal in an optical disk device, for example.
2. Background Art
Optical disks have almost reached their limit in terms of the resolution limit of the optical system with Blu-ray Disc (BD) by use of a blue-violet semiconductor laser and a high NA objective lens having been commercialized. A multilayer structure is expected to be dominant in the optical disks in the future to further increase the capacity. In a multilayer optical disk, the reflectance from a specific layer has to be kept small since the intensities of lights detected from respective layers are required to be substantially the same as each other. Meanwhile, in the optical disk, a transfer rate has also been continuously increased along with an increase in the capacity since it is necessary to increase the copying speed for videos or the like. If the transfer rate continues to be increased, a sufficient S/N ratio of a readout signal cannot be ensured. Thus, it is necessary to improve the S/N ratio of a detected signal to achieve the multilayer structure and the higher rate at the same time.
A technique regarding the improvement in the S/N ratio of the readout signal of the optical disk is described in JP Patent Publication (Kokai) Nos. 05-342678A (1993) and 06-223433A (1994), for example. Both of JP Patent Publication (Kokai) Nos. 05-342678A (1993) and 06-223433A (1994) relate to the improvement in the S/N ratio of the readout signal of a magneto-optical disk, and intend to amplify the amplitude of a faint signal by branching a light from a semiconductor laser before the light is projected onto an optical disk, combining a light which is not projected onto the optical disk with a reflected light from the optical disk to cause interference therebetween, and thereby increasing the intensity of the light which is not projected onto the optical disk. In differential detection between a transmitted light and a reflected light of a polarizing beam splitter that has been conventionally used in signal detection for the magneto-optical disk, the detection is performed basically by causing original incident polarization components and polarization components perpendicular to an incident polarization direction produced by polarization rotation by the magneto-optical disk to interfere with each other, and amplifying the perpendicular polarization components with the incident polarization. Accordingly, when the original incident polarization components are increased, the signal can be increased. However, in order to prevent data from being deleted or overwritten, the intensity of the light incident on the optical disk needs to be suppressed to a certain level or less. On the other hand, in the conventional techniques described above, a light caused to interfere with a signal light is separated from the signal light in advance, and the light is caused to interfere with the signal light without focusing the light onto the disk. Thus, the intensity of the light caused to interfere with the signal light for signal amplification can be increased regardless of the light intensity on the disk surface. Accordingly, in principle, as the light intensity is stronger within the allowable range of the intensity, a higher S/N ratio can be obtained in comparison with the noise of an amplifier for converting a photocurrent from a photo detector into a voltage, and the shot noise occurring in the photo detector.
In JP Patent Publication (Kokai) No. 05-342678A (1993), an interference intensity is detected by causing two lights to interfere with each other. The optical path length of the light which is not reflected from the disk and caused to interfere with the signal light is made variable, thereby ensuring the amplitude of an interference signal. In JP Patent Publication (Kokai) No. 06-223433A (1994), the differential detection is also performed in addition to the interference intensity detection. The intensity component of each light which does not contribute to the signal is thereby canceled, and the noise component of the light is canceled, thereby improving the S/N ratio. A non polarizing beam splitter is used for the differential detection in this case.